1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method, and more specifically, to correction of a formation position misalignment observed in an image formed in an electro-photographic development section or the like.
2. Description of the Related Art
In a recently provided electro-photographic color image formation apparatus is of a tandem type of color image formation apparatus, in which a development section including a development unit and a photoreceptor is equipped for each of plural types of color materials and images of different colors are sequentially on an image basis, transferred to an image transfer belt or a printing medium. With the development sections equipped for the respective color materials as such, image formation can be performed at high speed.
Although the time taken for image formation can be considerably reduced, such a tandem type has problems resulting from non-uniformity of a lens and position accuracy for attachment of the lens in a deflection scanning unit, and position accuracy for assembly of the deflection scanning unit itself to an image formation apparatus body. More specifically, when scanning lines on a photoreceptor are sloped or curved, and when the amount of slope or curve varies depending on the color, image formation positions of respective colors on a transfer paper may not be the same. As a result of this, the formed color image suffers from color shift due to formation position displacements among colors, and there thus is a problem of not being able to achieve the high quality for the color image.
As measures against such color shift, Japanese Patent Laid-Open No. 2002-116394 describes a method of measuring the amount of curve of scanning lines using an optical sensor in the assembly process of a deflection scanning unit, and adjusting, for fixation, the amount of curve of the scanning lines by mechanically rotating a lens. Japanese Patent Laid-Open No. 2003-241131 describes a method of measuring the amount of slope of scanning lines using an optical sensor in the assembly process of a deflection scanning unit to an image formation apparatus body, and adjusting, for fixation to the apparatus body, the slope of the scanning lines by mechanically tilting the deflection scanning unit.
For correcting an optical path in an optical system, there needs to mechanically operate components, such as an optical system including a light source and an f-θ lens and mirrors in the optical path, so as to achieve position alignment for a test toner image. As such, the methods of Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131 both require a high-accuracy moving member, thereby causing a cost increase.
Another problem with such optical path correction in the optical system is that no frequent correction is adapted due to the long time before completion of the correction. The length deviation of the optical path is changed due to the influence of temperature increase possibly occurred to the apparatus. Therefore, even with correction making at some point in time, it is not possible to eliminate the influence of temperature increase occurring to the apparatus. In this sense, it is thus relatively difficult to prevent color shift through correction for the optical path in the optical system.
On the other hand, Japanese Patent Laid-Open No. 2004-170755 describes a method of measuring the amount of slope and curve of scanning lines using an optical sensor, and correcting bitmap image data so as to cancel out the measurement results. More specifically, by performing coordinate conversion of an output of bitmap data, and by adjusting a gradation value of neighbor pixels, an image is formed with any possible slope and curve so that the slope and curve occurring in the apparatus are cancelled out. As such, the method of Japanese Patent Laid-Open No. 2004-170755 is of electrically correcting the bitmap image data through image data processing. Therefore, in terms of not requiring a member for mechanical adjustment or an adjustment process at the time of assembly, the method of Japanese Patent Laid-Open No. 2004-170755 can deal with the color shift at lower cost than the methods of Japanese Patent Laid-Open No. 2002-116394 and Japanese Patent Laid-Open No. 2003-241131.
Japanese Patent Laid-Open No. 8-85237(1996) also describes a method of correcting any color shift similarly to that of Japanese Patent Laid-Open No. 2004-170755. More specifically, this method executes image processing such as a color process and a halftone process to form raster image data on a bitmap memory for each of color components (C (cyan), M (magenta), Y (yellow), and K (black)). Then, the method converts output coordinates of the respective color image data into the output coordinates which has been subjected to a correction for registration deviation. Further, a light beam being a modulation result based on each of the conversion results, i.e., image data, is then corrected in units smaller than the minimum dot unit of a color signal.
However, if the corrections of color shift described in Japanese Patent Laid-Open Nos. 2004-170755 and 8-85237(1996) are simply performed at some point in the image processing process, the resulting images formed thereby may suffer from quality degradation contrary to expectations.
The correction of color shift is, as described in the methods of Japanese Patent Laid-Open Nos. 2004-170755 and 8-85237(1996), generally applied to image data that has been subjected to a halftone process. In the resulting image, halftone dots in the halftone image may show poor reproducibility due to the conversion of output coordinates and correction performed in units smaller than the minimum dot, thereby causing irregular color such as moiré.
FIG. 1 is a diagram illustrating such problems, and shows correction in units smaller than the minimum dot unit. An input image 101 is constant in density value (50%). The input image 101 is subjected to the correction of color shift in which an adjustment of gradation values around the minimum dot so as to execute data correction in units smaller than the minimum dot unit, as described in Japanese Patent Laid-Open Nos. 2004-170755 and 8-85237(1996). The resulting image data corrected as such is used as a basis for forming an image 102. The exemplary image 102 in FIG. 1 is the result corrected for color shift, in which the gradation value is shifted more in the righter side so as to accommodate the slope or the like of the scanning lines. More specifically, the image 102 formed based on the correction shows a line directed toward the upper right, and indicates that the color shift toward the upper left is cancelled out. With such data correction in units smaller than the minimum dot or conversion of output coordinates, halftone dots in the resulting halftone image may partially suffers from degradation of reproducibility, and thus a linear relationship can not be kept between density values of an image and density values of an image formed based thereon. If this is the case, even if the input image 101 is constant in density, an image formed based on the image data corrected for color shift may not be constant in density value. If such a density variation appears periodically, moiré problems become evident, and the resulting color image is not satisfactory.
On the other hand, it is considered that the correction of color shift may be performed before the halftone process. This, however, may cause a problem of not being able to keep the number of screen lines that are originally supposed to be processed due to a considerable angle deviation in a screen pattern, depending on an amount of color shift, then the quality degradation caused by not being able to keep the number of screen lines becomes more evident compared with the above case with the moiré problems.
In addition, a filtering process such as edge detection or smoothing is affected by the correction of color shift. On the other hand, the smoothing is generally required to be performed with higher accuracy if with an image high in output resolution. From this point of view, if with a high output resolution, when image data through with halftone processing is corrected for the color shift, the conversion of output coordinates accompanying therewith or the like prevents the filtering process from increasing in accuracy.
Further, a rendering processor the like is considered desirable if selectively performed to either halftone-processed image data or not-yet-halftone-processed image data, depending on as to whether quality of the output image comes first or speed increase and memory saving for the output image come first. In this case, it is desirable to perform correction of color shift depending on a mode for rendering, that is, if quality comes first in the mode, the correction of color shift is performed in the corresponding manner.